Like other industries, waste volume increases with the production scale in photo industry. As far as the amount of waste discharged from the industry is less than the threshold that can be managed by the natural environment, i.e., less than the environmental capacity, it does not generate any problem, however it does cause a problem when the amount exceeds the capacity. Therefore, it becomes necessary to reduce the quantity of wastes within the natural environmental capacity although it is proper that the wastes increase in quantity as the photographic industry prospers.
It is water pollution that becomes the major issue in the photographic industry. Recently, the standards of discharging silver into sewer have been regulated more severely with the years from the standpoint of preserving environment.
As for the regulations relating to the silver-discharge standards in the waste water discharged from a factory of photographic light-sensitive materials or the wash water of photographic processing, descriptions thereof can be found, e.g., in "Information Ecology and Safety" Regulations Affecting the Discharge of Photographic Processing Solutions, Kodak Publication No. J-102 (September, 1990). Table 1 thereof sets forth the value of 0.001-20 ppm as the silver-discharge standards of the sewerage regulation in a typical city; while the regulated value of silver in a photofinishing laboratory is reported as 0.5-5 ppm in Table 2 thereof. Some states of America are even of regulation that the discharge standards of silver should be less than 0.02 ppm.
Also, there can be found the regulated value of silver in the acts enacted in U.S.A. in 1986, such as "Safe Drinking Water Act", "Hazardous Materials Transportation Act", "Toxic Substances Control Act" and so on. Further, the regulations of sewer in U.S.A. becomes severe in many regions in response to the operation of "Revision of Clean Water Act" in 1989.
In addition, Europe also has severe regulations, and the standards of discharging silver into sewer is not more than 0.1 ppm.
Accordingly, it has been desired to invent a method of removing silver to such an extent as to satisfy the severe standards cited above, especially an effective method of reducing a silver concentration to 1 ppm or less, and discover an art of efficiently removing silver while keeping down the cost of a disposal plant which can cope with the severe discharge standards.